Haze and reduced visibility are the effects of air pollution most noticed by the general public, and are often thought of by the public as a measure of air quality. The adverse effects of air pollution on the clarity of the air are especially undesirable in remote wilderness areas and in the National Parks, where the scenic vistas are an important part of the visitor's experience. The U.S. Congress has sought in the 1977 Amendments to the Clean Air Act to preserve visibility, and set as a national goal "the prevention of any future and remedying of any existing impairment of visibility in any mandatory Class I Federal area which impairment results from manmade air pollution." Class I Federal areas include most of the national parks, larger wildernesses, and international parks. To obtain the technical data necessary to learn about the origins of visibility reducing hazes and to monitor progress towards achieving this goal, it is necessary to measure the optical properties of our atmosphere which relate to visibility. It is desirable to have a simple, reliable, cost effective method for monitoring visibility, which is practical for use even in remote areas.
Visibility monitoring is performed by government agencies to obtain data to protect the public interests. Monitoring must also be performed by industrial operations to understand the effects of their emissions and to obtain data which may be submitted in support of permit applications.
In recent years, most visibility monitoring in remote areas has been done by the measurement of the contrast of distant terrain features against the horizon sky (Malm, 1979; Malm et al., 1981; Tombach et al., 1987). An analysis of pairs of measurements taken close to each other has shown that this measurement method gives results which show considerable scatter, even when the sky is relatively free of clouds (White and Macias, 1986). This outcome can easily be explained by the variable illumination of the target and the sight path caused by many factors, including the variations in the reflectance of the ground (Richards, 1987). Because of the significant uncertainties in transmittances calculated from contrast measurements, there is a need for a better monitoring method.
In an attempt to fill this need, the National Park Service has sponsored the development of a transmissometer. The resulting instrument (Malm et al., 1986) is expensive, and does not provide data for the path radiance, which is shown below to be an important omission.
Photographic methods have long been used for visibility monitoring (Middleton, 1958; Steffens, 1949; Roberts and Gordon, 1974; Johnson et al., 1985). They have the advantage that photographic equipment is inexpensive to purchase and to operate unattended. Photographic visibility monitoring methods in current use typically attempt to measure the contrast of distant terrain features against the horizon sky. The demonstration by White and Macias (1986) of the unreliability of visibility data derived from instrumental measurements of contrast, which are of higher quality than can be obtained from photographs, shows that the contrast measurements derived from photographic data must also be unreliable.
This invention fulfills the need for a simple, reliable visibility monitoring method which uses equipment of reasonable cost and which can be easily used in remote locations. It is an important feature of the invention that both the transmittance and path radiance of sight paths are measured.